1. GCSE OCR 4 Storage Computer Science J276 Unit 1
System architecture, memory and storage 4

2. Objectives Describe flash memory
Discuss the need for secondary storage including optical, magnetic and solid state storage
Evaluate suitable storage devices and storage media for a given application using the following characteristics:
capacity,
speed,
portability,
durability,
reliability,
cost

3. Secondary storage
What do you already know about secondary storage and its role within computer systems?
MAIN MEMORY
SECONDARY
STORAGE
CENTRAL PROCESSING
UNIT (CPU)
INPUT
DEVICES
OUTPUT

4. Secondary storage
Are secondary storage devices volatile or non- volatile?
Why do we need secondary storage?
Why are there so many different types of secondary storage?

5. Uses of secondary storage
Secondary storage is non-volatile – it keeps its content when the power is switched off
It has many different uses – for example:
Programs and data are stored on hard disk
CDs may be used to distribute software, music, e-books etc.
Memory sticks may be used to transport data from one place to another
Magnetic tape or external hard drives may be used for backup
What other uses are there for secondary storage devices?

6. Storage types Primary storage Secondary storage
RAM and ROM
Secondary storage
Hard Disk Drive (HDD)
Solid State Drive (SSD)
Offline secondary storage
Compact Disc (CD) or Digital Versatile Disc (DVD) or BluRay
USB Flash memory
Removable HDD
Magnetic tape

7. Storage methods
Magnetic: Mechanical parts move over the disks surface to read and write data magnetically
Optical: Lasers read and write data using light
Solid State: Data is recorded onto solid memory chips without any moving parts

8. Magnetic disks Basic features:
Disk contains concentric circles called tracks
Each track is divided into sectors
Disk heads mounted on mechanical arms read and write the data

9. Inside a hard disk

10. Magnetic storage: hard drives
Fixed hard drives are built into virtually all PCs and laptops
They have a very large storage capacity, up to 6TB or more
Portable hard drives can be connected to a computer via a USB port
They are used for backing up or transporting data
Some portable music players have tiny hard drives no bigger than a small coin
They can store several GB of data

11. Magnetic storage Advantages: Disadvantages: Uses: Capacity:
Cheap, vast storage capability, fast write speed
Disadvantages:
Lots of mechanical parts, durability an issue, sealed unit due to disk head and platter precision and not very portable
Uses:
Personal computers, storage of vast quantities of data
Capacity:
500GB - 6TB or greater

12. Optical storage Basic features:
Data is stored as pits and lands burnt into a spiral track circulating outwards from the centre
A laser beam passes over the pits and lands the level of reflection is measured
From this signal, 0s and 1s can be derived from reflections or no reflections

13. How CDs work

14. Optical storage Advantages: Disadvantages: Uses: Capacity:
Cheap, very easily portable, takes up little space physically
Disadvantages:
Less storage capacity compared to other types
Easily damaged / scratched, requires a CD reader
Slow write speeds
Uses:
Songs, videos and other multi-media storage, backup and archiving of data
Capacity:
CD-ROM – up to 720Mb
DVD – up to 8.4Gb
Blu-Ray – up to 50Gb

15. CDs, DVDs and BluRay
Why are the capacities of these discs different given they are all the same physical size?
Microscopic view of the surface of a CD Rom

16. Pit size and laser wavelength

17. Solid State Drives (SSD)
Basic features:
Solid-state disks use non- volatile flash memory to store information
Very fast burst read/write speeds due to data being physically close and easy to recover
No mechanical or moving parts

18. Solid State Drives Advantages: Disadvantages: Uses: Capacity:
Highly durable, no moving parts, very fast read/write speeds, no noisy fan, faster start up times
Disadvantages:
Expensive at present, less storage capacity / physical size than traditional hard disks
Uses:
Hand held computers
Military usage
Capacity:
100GB – 16TB

19. Flash memory Low cost, portable, no moving parts, durable
This makes them ideal for a range of offline devices:
Cameras
Mobile phones
USB memory sticks

20. Electrons trapped in this layer
How flash memory works
Large electric current used to force electrons through a barrier and trap them on the other side
They remain on the other side until “flashed” with a new current, hence the name
Trapped (charged) or not trapped = 0 or 1
Electrons detected here
Barrier
Electrons trapped in this layer
Current applied and electrons forced through barrier

21. Cloud storage
Cloud storage refers to saving data in an off-site storage system maintained by a third party
Examples include example Dropbox and Google Drive
Instead of saving data on your computer's hard drive or other local storage device, you save it to a remote database, and access it via the Internet
What are the advantages in doing so?

22. Worksheet
Complete Task 1 on the worksheet

23. Data capacity
Different storage devices have a range of storage capabilities
A higher capacity will allow greater data storage
How do you decide which storage device to use?
How do you calculate storage requirements?

24. Data capacity
When you know how much data you need to store, you can decide which storage device(s) would be most appropriate
If you have 300GB of data to store, is optical storage appropriate?
If not, why not? What could you use instead?
Why is this particular device appropriate?

25. Calculating data capacity
Knowing the capacity required will enable us to make an informed decision as to which device to use
If we have 500 files that are 300MB each, we need a total of 150GB
Which storage devices are suitable for this amount of data?

26. Worksheet
Complete Task 2 and 3 on the worksheet